It is well known that catalyst component with magnesium, titanium, halogen and electron donor as main ingredients can be widely used in the polymerization of olefins, especially in the polymerization of ethylene or co-polymerization of ethylene and alpha-olefin(s). When such a catalyst component is used in fluid bed gas phase polymerization process, the catalyst component is typically supported on a carrier, such as silica and the like, in order to ensure the morphology and particle size distribution of the catalyst particles to be suitable for the operation of fluidized state. For example, the catalysts for fluid bed gas phase process disclosed in U.S. Pat. No. 4,302,565, U.S. Pat. No. 4,379,759 and CN 1064870A(corresponding to EP0499093) are prepared by impregnating a mother component formed from a titanium compound, a magnesium compound and an electron donor compound onto a carrier, such as silica and the like, and treating the impregnated mother component with an activating compound. The silica used in U.S. Pat. No. 4,302,565 and U.S. Pat. No. 4,379,759 have an average particle size of from 50 to 150 microns, a surface area greater than 50 m2/g, and an average pore size greater than 80 angstroms.
In the above mentioned patents, in order that the catalyst is suitable for fluid bed gas phase process for polymerizing ethylene, there is relatively strict requirement in average particle size, surface area and pore size of the carrier, i.e. silica used. In addition, the carrier material is expensive, and the silica carrier needs to be activated strictly prior to use. Thus the cost of the catalyst is high. Furthermore, since the catalyst active component is supported on carrier by a process such as impregnation and the like, the catalyst efficiency is not satisfactory. Further, the fine powder content of the polyethylene powder produced using said catalyst is relatively larger so that the particles having a particle size less than 150 microns comprise typically about 15 percent by weight of the whole powder, and this is very undesired in commercial production. When the catalyst is used in a plant for gas phase fluid bed ethylene polymerization, it is usually fed in the form of solid so that the stability in the feeding step is poor and phenomena such as blockage and bridge formation in pipelines are easy to occur.
Chinese patent CN 85100997(corresponding to U.S. Pat. No. 4,784,983) disclosed a catalyst for olefin polymerization, which was prepared by dissolving a magnesium halide into an organic epoxy compound and an organophosphorus compound to form a homogeneous solution and reacting the solution with at least one precipitation aid, a polycarboxylic ester electron donor and titanium halide or derivative thereof. The catalyst exhibits high polymerization activity and good stereoregularity when it is used in propylene polymerization. However, when it is used in ethylene polymerization, the catalyst exhibits low polymerization activity, broad distribution of particle size of polymer and poor hydrogen response.
On the basis of above mentioned Chinese patent CN 85100997, EP 1083187A1 disclosed a catalyst for ethylene homopolymerization or co-polymerization, which was prepared by dissolving a magnesium halide into an organic epoxy compound and an organophosphorus compound, adding an electron donor activator thereto to form a homogeneous solution, and reacting the solution with at least one precipitation aid and titanium halide or derivative thereof. The catalyst exhibits relatively high polymerization activity and the polymer obtained is good in particle morphology and high in bulk density, when the catalyst is used in slurry polymerization of ethylene. However, when the catalyst component comprising alcohol type of activator is used in gas phase process of ethylene polymerization, especially in fluid bed process of ethylene polymerization, the results are not satisfactory. The rate of the polymerization reaction is relatively quick, especially in the initial stage, so that the polymer particles produced are fragile and the polymer particles are relatively fine. In general, the polymer particles having a particle size of from 70 to 150 microns comprise from 50 to 60 percent by weight of the whole powder. In addition, the morphology of the polymer particles and the flowability of the polymer powder are poor.
Therefore, there exists a strong need for providing a high efficient solid catalyst component, which is capable of producing polymer having low content of fine powder, has relatively narrow distribution of particle size and appropriate average particle size, is high in catalytic activity, and is very suitable for ethylene slurry or gas phase polymerization, especially for fluid bed gas phase process of ethylene polymerization in which catalyst is fed in the form of slurry.
It has been found that a catalyst component, which has appropriate average particle size, relatively narrow distribution of particle size, good particle morphology, particles of which are not easy to break during polymerization reaction, and which therefore is very suitable for ethylene homopolymerization or co-polymerization, especially for gas phase process of ethylene polymerization, can be obtained by supporting a suitable electron donor and optionally a titanium compound onto a composition comprising magnesium and titanium as carrier component, or by supporting a suitable electron donor and a titanium compound onto a magnesium compound-containing composition, and optionally treating the product containing magnesium, titanium and electron donor with an activator, without using silica and the like as carrier. Since the catalytic activity of the catalyst according to the present invention is also improved greatly, it is possible to dilute the catalyst with a inert diluent and achieve homogeneous feeding in liquid state by means of pumping, thereby overcoming the problems encountered in the above catalyst feeding step.